The present invention relates to a belt retractor comprising a locking pawl actuated by an electromagnet.
In order to move the locking pawl from its resting position into the engaged position with the requisite speed and reliability, the electromagnet has to generate a substantial amount of force. For this purpose, it is dimensioned for the force needed to activate the locking pawl. If a current is applied to the electromagnet for a prolonged period of time, it heats up considerably. For the electromagnet to be able to generate the necessary actuation force and to withstand the thermal stress under continuous load, it must be quite large in size.
The invention reduces the load on the electromagnet so that it can be made smaller, as a result of which the volume and weight of the belt retractor are ultimately reduced.
According to the invention, the electromagnet is operated with a holding power that is reduced in comparison to its actuation power The actuation power for moving the locking pawl from its resting position into the engaged position is only provided for a short period of time, subsequently, it switches over to a reduced holding power. The criterion for switching over from the actuation power to the reduced holding power can be the lapsing of a predetermined period of time As an alternative, the heating up of the electromagnet is detected and the device switches over to the reduced holding power when a predetermined temperature is exceeded It is especially advantageous to detect the position of the locking pawl and, after the engaged position has been reached, to immediately switch over to the reduced holding power However, electric parameters, such as the current flowing through the magnetic coil, also provide information on the actuation procedure, so that these parameters are also options as a criterion.
Several embodiments are provided for operating the electromagnet with the reduced holding power.
According to a first embodiment, a voltage regulator reduces the voltage supplied to the electromagnet during the holding phase The voltage can also be reduced by means of pulse duration modulation According to another embodiment, the electromagnet is provided with two windings that can be actuated separately, one of them, which is provided as the holding winding, being more weakly dimensioned.